So Bard can stun his entire team?...
I think there should be some sort of in game thing that you should have to click to be affected by Bard's ult if he's on your team, which can also be cancelled at any time. The potential to troll with his ult is insane. My marksman is a jerk, I'll stun him and walk away. Enemy team is chasing, I'll stun my entire team and walk away. Enemies are diving a tower, I'll stun my tower. These are a few ways that I'd imagine it could be abused. Personally, if I'm in a team fight, I don't want my own teammate randomly tossing an AOE stun onto me.
I'm also curious if his ult works on the Obelisk.
EDIT: Relax everyone, I know it's a "stasis". Even though you're invulnerable, you're still unable to move or attack, so you're basically stunned. Being stunned is being unable to react. I know it's not classed as a stun by Riot, but by actual definition you are stunned since you can't react. The entire point of this is concern for abuse, don't be eager to try and correct someone.
EDIT2: Also, this is a legit possibility and concern. I would bet my life on players using his ult to troll. Please don't down vote me for being concerned about the quality of my games.